Rechargeable electrochemical cells, such as nickel-cadmium cells, have been assimilated to a wide range of end uses. Such cells are now commonplace not only in industrial environments, but on the mass consumer market. As the scope of use has expanded, so has the likelihood of misuse and misapplication of the cells by the user, either during the discharge or charge cycle.
Among the problems caused by excessive current flow through the cells are those of cell degradation or, in the case of malfunction of a vented cell, dangerous overpressure and overtemperature conditions in the cell. These conditions present the risk of damage being caused to the cell or to equipment in electrical connection with the cell. Indeed, these conditions may even present the possibility of injury being caused to a user. Such problems are amplified by the use of several interconnected electrochemical cells which produce higher voltages and higher current drain capacity. Moreover, the use of several individual cells in a single battery unit (cell pack) increases the probability of malfunction.
Encasing the cell pack of electrochemical cells in a material capable of withstanding appreciable amounts of heat and pressure can provide a solution to these difficulties. However, such a casing would detract from the overall convenience and efficiency of the cell pack. Fusing the cell pack by conventional techniques such as placing a fuse in the vicinity of the power source or permanently soldering a fuse into place also presents difficulties because the portable nature of the cell packs requires a compact source structure which can be readily maintained.
Although most cells are equipped with small built-in pressure release valves to vent internal gases in the event of an overpressure condition, the possibility still remains that an overpressurized cell may cause damage to the cell pack or to the equipment the cell pack powers or is being charged with. Moreover, cell performance can be seriously downgraded in the event of such overpressure conditions. For example, during a venting period early in the cell life, some of the electrolyte in minature droplet form can be entrained in the vented gas stream. Should repeated venting occur due to pressure increases at high current loadings, a significant loss of electrolyte and gases is likely.
Excessive cell temperature conditions are aggravated whenever a plurality of cells are grouped together, since each cell is in effect a heat generating source. When placed in a common container or package, the rate of dissipation of internally generated heat to the environment is reduced, and the cells operate at higher equilibrium temperatures. Such temperatures are directly related to the amount of current flowing through the cell during charging and discharging. As current increases, heat generated through the cell's internal resistance R.sub.c increases according to the formula I.sup.2 R.sub.c. Not only do excessive temperatures promote excessive cell pressures and the risk of damage or injury incident thereto, prolonged exposure of the cell to high temperatures hastens decomposition of the separator and seal materials.
The foregoing considerations highlight the need for an effective means of overcurrent protection which will preclude excessive current flow through a multi-cell battery pack.
Among the objects of the invention, therefore, is to provide a rechargeable electrochemical cell pack having circuit breaker means for protecting the cell pack and its surrounding equipment against overcurrent conditions.
Another object of the invention is to provide rechargeable electrochemical cell packs having circuit breaker means which can be manufactured cheaply and in a compact configuration which does not increase the overall dimensions of the unit.
These and other objects will become apparent from the following summary and description of the invention taken in conjunction with the accompanying drawings.